As is known in the art, a helicopter flying at cruise speeds (for instance, in excess of 50 knots) accomplishes turns, for a change in heading, by inducing a bank angle and undergoing a commensurate heading change rate as a function of speed. This maneuver is equivalent to a pull-up maneuver in terms of loads induced in the helicopter, particularly main rotor blade loading. This is due to the force necessarily applied to the helicopter through the blades in order to effect the required directional acceleration against the mass of the helicopter, and in a pull-up, to also overcome the acceleration of gravity. In fact, a 60.degree. level bank angle (which is not uncommon) will nominally double the loading on the main rotor.
As is known, a helicopter has a maneuvering neutral point (which may be thought of as a dynamic center of the helicopter, as far as maneuvers are concerned). When the helicopter is loaded in a fashion that the weight distribution centroid (or center of gravity) is forward of the neutral point, the helicopter is relatively stable in maneuvers. But when the center of gravity is at or aft of the neutral point, the helicopter is relatively unstable in certain maneuvers such as banked turns and pull-ups. This is believed to be due to the fact that the excess weight, aft of the neutral point, tends to rotate around the neutral point whenever the helicopter undergoes any change in direction of flight, much the same as a rear-engine automobile has a greater tendency to skid when in a turn. Consider a pull-up maneuver: as the nose pulls up, the velocity vector of the excess weight aft of the neutral point is no longer in line with the neutral point, so the weight tends to induce a tail-down rotation of the helicopter, so that a commanded nose-up rate is accompanied by an additional weight-induced nose-up rate, which continues until the helicopter resumes straight-line flight (that is, until the velocity vector of the aft weight is through the neutral point again). Also, this undesirable pitch rate may be induced by gusts, etc. Since a banked turn is the same (in the helicopter pitch axis) as a pull-up, the same effect occurs. Thus, a helicopter having excess aft weight is unstable in maneuvering about its pitch axis in pull-ups and banked turns.
In helicopters, any nose-up maneuver loads the main rotor in proportion to the pitch rate, as sensed by the pitch rate gyro. The pitch rate is induced by pulling aft on the cyclic pitch stick, which requires exerting a force on the stick proportional to the displacement thereof. Therefore, the degree of loading of the helicopter as a consequence of a maneuver can be sensed by the feel of the stick. However, when the helicopter has maneuvering instability in its pitch axis, the loading caused by the weight-induced pitch rate is not sensed in the stick. And, if the stick is returned to neutral longitudinal cyclic pitch, the helicopter will still have a weight-induced pitch rate; the only way to remove the weight-induced pitch rate is to go past neutral to command an equal nose-down rate, to return to straight-line flight (in pitch).
Thus, a helicopter with its center of gravity aft of the maneuvering neutral point will respond more than is desired, requiring nose-down (forward) cyclic stick motion to counteract the weight-induced pitch rate. This therefore gives the pilot no feel at all, or an unloading feel. If uncompensated by the pilot's stick motion, any pull-up or turn will grow increasingly tight, called "digging-in", with increasingly dangerous rotor loading, which the pilot cannot feel. Naturally, the effect of the instability is nil at low speeds and increases with speed to the degree that it may be intolerable at high speeds.